RFR: 8257466: Improve enum iteration

[Ioi Lam]

On Tue, 1 Dec 2020 04:41:35 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this collection of improvements to the recently added enum
> iteration facility. These improvements are based on usage in some
> in-development changes.
> 
> (1) Added EnumType nested type to EnumRange and EnumIterator. It is an alias
> for the enum type that is a template parameter for those classes. This is
> useful when dealing with a range or iterator whose associated enum type is
> not known because the range/iterator is a template parameter.
> 
> (2) Added EnumRange<T>::index(T), which converts the argument enumerator to
> a zero-based index into the range of values. This is useful when mapping
> from an enumerator to a corresponding array index, for example.
> 
> (3) Allow enum range bounds to be specified using start and (exclusive) end
> integral (underlying type) values. This is useful when dealing with enums
> that are just value ranges and don't have named enumerators. As part of
> this, changes the enum iteration traits mechanism to use start/end rather
> than first/last, as that seems to be a little easier to deal with.
> 
> (4) Added accessors for the first and last enumerator values of a range.
> 
> (5) Added gtest for enum iteration.
> 
> Testing: tier1

Marked as reviewed by iklam (Reviewer).

src/hotspot/share/utilities/enumIterator.hpp line 115:

> 113:     static constexpr EnumeratorRangeImpl::Underlying<T> _start =        \
> 114:       EnumeratorRangeImpl::underlying<T>(Start);                        \
> 115:     static constexpr EnumeratorRangeImpl::Underlying<T> _end =          \

Why do we need to call `underlying<T>(Start)` here? Can it be simplified as 
static constexpr EnumeratorRangeImpl::Underlying<T> _start = Start;
or
static constexpr EnumeratorRangeImpl::Underlying<T> _start {Start};

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PR: https://git.openjdk.java.net/jdk/pull/1530